Optical filters are fundamental elements in many optical instruments and setups. In many optical techniques, such as but not limited to, fluorescence spectroscopy, fluorescence anisotropy and Raman spectroscopy, it is important that the excitation light be spectrally distinct from that spectral portion of the fluorescence light that is used for detection. Optical filters that exploit thin-film interference effects to spectrally filter the light are commonly used to filter the excitation light and the fluorescence light.
Linear polarisers are optical elements that, ideally, only transmit one component of the input electromagnetic radiation, where the component of the electric field that is transmitted is oriented parallel to the axis of the polariser.
In addition to these two properties, may also be desirable for an optical element to achieve angular filtering, whereby in order to be transmitted by the device, photons of the selected component of polarizations must not only be within the designed spectral transmission envelope, but it must also be propagating within a given range of angles with respect to the entrance face of the optical element. Spatial filtering is another term that is commonly and synonymously used for angular filtering. Angular filtering is commonly achieved by placing apertures in different places along the direction of beam propagation. This adds to the size and complexity of the device because achieving a high degree of angular filtering requires a long optical path. Hard apertures also cause unwanted diffraction effects. By combining these three common tasks of spectral filtering, angle filtering, and polarization, into a single unit, the design and complexity of many optical instruments and apparatuses can be reduced.
Tunable filters that are based on diffraction gratings are widely used in spectrometers. The peak transmission wavelength of these devices can cover a wide range, but for a given range the bandwidth can only be narrow if the size of the device is large. Another drawback for some applications is that the transmission at the peak wavelength tends to be fairly low, although this can be mitigated in limited spectral regions by blazes and holographic gratings. Other types of filters have been developed for many of these applications. Birefringent (Lyot) filters are intermediate in size and bandwidth. Fabry-Perot filterscan be quite small, but are limited in bandwidth, or more precisely in the finesse, which is the ratio of the isolation range to the wavelength width of the transmission range. Tunable thin film filters based on multi-cavity designs have been used in telecommunications. These can be small, and the tuning range is limited. Thin film filters can also be based on Rugate designs.
Tunable filters based on spatially-varying thin film designs have been developed, as for example in U.S. Pat. No. 6,700,690, “Tunable variable bandpass optical filter,” Philip E. Buchsbaum et al., Mar. 2, 2004. The tunable filter described in U.S. Pat. No. 6,700,690 is based on a traditional filter design that is based on Mirror structures and Fabry Perot spacer layers. The background material in U.S. Pat. No. 6,700,690 nicely describes other similar tunable filters. Turner discloses a single waveguide filter with frustrated total reflection interference in U.S. Pat. No. 2,601,806.